Pattern approval for measuring instruments

Courtesy: Pattern approval for measuring instruments

The second letter indicates the frequency weighting. “Pattern approved” sound level meters typically offer noise measurements with A, C and Z frequency weighting.

Z-weighting represents the sound pressure equally at all frequencies. A-weighting, weights lower and higher frequencies much less, and has a slight boost in the mid-range, representing the sensitivity of normal human hearing at low (quiet) levels. C-Weighting, more sensitive to the lower frequencies, represents what humans hear when the sound is loud (near 100 dB SPL).

The IEC 61672-1:2013 mandates the inclusion of an A-weighting filter in all sound level meters, and also describes C and Z (zero) frequency weightings. The older B and D frequency weightings are now obsolete and are no longer described in the standard.

In almost all countries, the use of A-weighting is mandated to be used for the protection of workers against noise-induced hearing loss. The A-weighting curve was based on the historical equal-loudness contours and while arguably A-weighting is no longer the ideal frequency weighting on purely scientific grounds, it is nonetheless the legally required standard for almost all such measurements and has the huge practical advantage that old data can be compared with new measurements. It is for these reasons that A-weighting is the only weighting mandated by the international standard, the frequency weightings ‘C’ and ‘Z’ being options.

Originally, the A-weighting was only meant for quiet sounds in the region of 40 dB sound pressure level (SPL), but is now mandated for all levels. C-weighting is however still used in the measurement of the peak value of a noise in some legislation, but B-weighting – a halfway house between ‘A’ and ‘C’ has almost no practical use. D-weighting was designed for use in measuring aircraft noise when non-bypass jets were being measured; after the demise of Concord, these are all military types. For all civil aircraft noise measurements, A-weighting is used, as is mandated by the ISO and ICAO standards.

If the third letter is F, S or I, this represents the time weighting, with F = fast, S = slow, I = impulse. Time weighting is applied so that levels measured are easier to read on a sound level meter. The time weighting damps sudden changes in level, thus creating a smoother display.

The graph indicates how this works. In this example, the input signal suddenly increases from 50 dB to 80 dB, stays there for 6 seconds, then drops back suddenly to the initial level.

A slow measurement (yellow line) will take approximately 5 seconds (attack time) to reach 80 dB and around 6 seconds (decay time) to drop back down to 50 dB. S is appropriate when measuring a signal that fluctuates a lot.

A fast measurement (green line) is quicker to react. It will take approximately 0.6 seconds to reach 80 dB and just under 1 second to drop back down to 50 dB. F may be more suitable where the signal is less impulsive.

The decision to use fast or slow is often reached by what is prescribed in a standard or a law. However, the following can be used as a guideline: The slow characteristic is mainly used in situations where the reading with the fast response fluctuates too much (more than about 4 dB) to give a reasonably well-defined value. Modern digital displays largely overcome the problem of fluctuating analogue meters by indicating the maximum r.m.s. value for the preceding second.

An impulse measurement (blue line) will take approximately 0.3 seconds to reach 80 dB and over 9 seconds to drop back down to 50 dB. The impulse response, I can be used in situations where there are sharp impulsive noises to be measured, such as fireworks or gunshots.